On any Euro banknote from the first series (2002), find the digital root of the serial number's digits (forget the initial letter).

The digital root is the value obtained by summing all the digits, then if the resulting sum has more than one digit summing these digits again, and so on, until a single-digit value is attained. "For example, the digital root of 65,536 is 7, because 6 + 5 + 5 + 3 + 6 = 25 and 2 + 5 = 7."

Here's a riddle: what is the value (from 1 to 9) you can never obtain on any Euro banknote from the first series (2002), when computing its digital root? In other words, what is the impossible digital root on Euro banknotes from the first series?

Here's a second riddle: on a (Europa series), what is the impossible digital root?

lmviterbo wrote:Here's a riddle: what is the value (from 1 to 9) you can never obtain on any Euro banknote from the first series (2002), when computing its digital root? In other words, what is the impossible digital root on Euro banknotes from the first series?

Here's a second riddle: on a (Europa series), what is the impossible digital root?

1) Yes. The serial letter would have to be either I or R in that case.

2) No. 8 is the digital root in serial numbers on PA, UE and YA tenners. Theoretically, yes, all digital roots will eventually be possible. I don't think that's bound to happen in the near future though.